The invention concerns a method for the production of optical components, particularly those of a crystalline base material, with an increased stability as well as the components obtained thereby and their use.
Optical components, such as lenses and prisms, have been known for a long time for the production of different optics for multiple purposes of application. They are usually comprised of a base material that transmits electromagnetic radiation, and this material has an optically active three-dimensional shape or geometry, which deflects the direction of the wave beam that passes through it at its surfaces when it enters and exits. It usually happens that—insofar as possible—the optically active shape is cast directly in a pattern predominantly in the case of plastics, or in the case of optical glass or crystals is cut out of a block of material by chipping, sawing and grinding.
The surface of such optical elements, however, is not completely flat, but has microscopic roughness of variable size, which is also designated as micro-roughness. Now if a light ray impinges on such a rough spot, then it enters into an interaction with the optical element at a different entry or exit angle than a light ray striking a completely flat surface. Therefore, this ray or the electromagnetic wave is deflected in an undesired direction corresponding to the modified entry/exit angle. In this way, a light scatter arises, which is typical for each optical component or lens and which adversely affects its power.
Many investigations have already been undertaken in order to smooth the surfaces of such optical components. Since the micro-roughness of optical surfaces cannot be randomly reduced by means of grinding, polishing and etching, it has already been attempted to smooth these roughnesses by depositing a dielectric material with the same or approximately the same refractive index. As is described in DD (East Germany)-A 288,466, the dielectric layer is introduced, for example, by means of reactive electron-beam deposition. In this procedure, however, only as much material is deposited on the surface that the depressions or the troughs of the micro-roughness are more or less filled, whereupon a smoother surface results.
This method, however, assumes that the optical surface is already optimally polished prior to the deposition of the dielectric layer, i.e., an optimal or minimal micro-roughness has been produced. In order to produce such a highly polished minimal micro-roughness, however, a number of processing steps are necessary. Each processing step, however, exercises additional stress on the molecular framework, particularly on the crystal structure of the base material. In addition to the already present crystal or material defects, this stress leads to additional micro-disruptions in the molecular structure of the base material or leads to unloading of stresses and strains in the material, which are caused, e.g., by crystal defects that are already present. The stability of the base material is reduced thereby. If the material has crystalline structures and particularly if it is a single crystal, then it may happen that the crystal is disrupted by slight temperature stress or mechanical loads or shifts in fits and starts along a crystal surface to break down the stresses or strains present in the crystal unit. By the processing of such materials, particularly by grinding and polishing, crystal defects reach the surface due to the abrasion of material associated therewith, so that these crystal defects can unload their stress or strain with a release of energy, which leads to the previously described shift or even to the breaking off of pieces of material of variable size. Such effects are particularly pronounced in soft and highly pure crystals. Foreign atoms introduce defects in highly pure crystals, because their inappropriate size for the most part gives rise to small glassy regions. These glassy regions act as a putty, which macroscopically prevents a sliding of a lattice. Due to the transmission requirement as well as the requirement for maximal homogeneity and minimal strain birefringence, crystals for optical lithography in the UV must be highly pure.
The above-described material defects bring about a further reduction in yield in the production of optical components. If one additionally considers the reduced yields for large-scale technological processes, in any case, in the growing of large, highly homogeneous, oriented single crystals, as are necessary for optical components with a large diameter, such as, for example, in objectives for microlithography, then an increase in the number of discards during processing or production of such optical components leads to an additional enormous reduction in the yield of the total process, which greatly increases costs.
Not only is the minimizing of light scatter of importance, however, for the quality of such high-performance optics, as well as their components or their optical elements, but also their optically active three-dimensional shape or their geometry. It is attempted to adapt the actual shape of the optical component as close as possible to the theoretical shape. This has been done conventionally by producing a negative pattern corresponding to the optimal or theoretical lens or prism shape, in which the finished lens or prism is inserted. Minimal deviations from the optimal fitted shape are then shown as Newtonian rings. The fewer the Newtonian rings that are observed, then the better a finished lens is adapted to the negative shape, i.e., it comes closer to the desired optimal uniform lens curvature or prism angle. The quality of the lens shape is thus also called “fit accuracy” or “simply fit”. More modern measurement methods operate without contact by means of interferometers. The shape of the surface is determined in a highly accurate manner with suitable computer evaluation techniques by means of a wave, which is reflected at the test piece, and which is brought to interference with a reference wave.
It has now been found in crystals that by polishing to a small micro-roughness in an attempt to prevent light scatter, a good fit cannot be achieved. On the other hand, if it is attempted to obtain a good fit by intense polishing, then an intensified micro-roughness of the surface occurs and thus there are considerable losses due to light scatter.